The present disclosure relates to a method, apparatus, and computer program product for optimizing inspection recipes using a mask with programmed defects.
Determining, optimizing, and monitoring the sensitivity of optical, laser-based, electron-beam and other inspection tools are of great importance for state-of-art semiconductor process technologies. The sensitivity is typically defined by the size of the smallest defect that the inspection can detect.
One approach for optimizing the sensitivity of an inspection tool utilizes an arbitrary production wafer. Initially, an inspection threshold is set to a very low level so that lots of noise is picked up as well as all of the detectable defects based upon the current inspection tool settings. The smallest defect detected may or may not be the smallest defect that exists on the wafer. The inspection may then be optimized by iteratively adjusting the inspection conditions so that the signal of these defects is stronger in order to identify any additional defects. Once optimized, this same production wafer can then be used to monitor the inspection stability by running it again at a later date.
This approach has a number of drawbacks. First, a range of the appropriate sized defects may not exist on every production wafer. As a result, many of these wafers are not good candidates for recipe optimization. If a poor candidate wafer is used, sub-optimal inspections may result. Second, valuable inspection tool time may be consumed evaluating whether a wafer is a good candidate. Finally, each wafer has a finite life for inspection recipe monitoring, making long term monitoring difficult.
One technique for addressing this problem is to utilize programmed defects; that is, intentional defects embedded into the design of a product-like test structure. In a good design, the programmed defects will span the range of interest. Because they are part of the design, they will appear in every reticle field and on every wafer. Therefore, a different wafer can be used every time for long term monitoring.
One disadvantage of this approach is that it is suitable only for array mode inspections, where potential defects are compared to reference sites 2 um to 50 ums away within the same array. Many applications are not suited to array mode inspection and instead require random mode inspection, where the potential defect image is compared to a reference image in the same position on the next die. Logic circuitry is one example where random mode inspection is preferred. Also, for a large area inspection, random mode may be the appropriate choice, since, although some regions within this area have a pattern, no single offset will apply for the entire area. With random mode inspection, however, programmed defects would not be detected because they appear in the same place within each flash field.
One solution to this problem is to utilize two masks for each level of inspection, where the second mask is the same as the first except that it contains the programmed defects. These masks are then alternately flashed across the wafer so that every third reticle field would contain the programmed defects. To differentiate the good die from the bad die, double arbitration may be used. The defective die is compared to its neighbors to the left and right. The main drawback to this approach, however, is that it requires the purchase of an additional reticle per layer of interest which is very expensive. Generating wafers of this type is also expensive in terms of scanner time and only a few wafers would likely be generated.
Another alternative is to place three similar test structures equally spaced across the flash field. One of these structures would contain a programmed defect. Inspection tool sensitivity in random mode could be optimized by using a pitch equal to one third of the reticle field. The disadvantages of this approach are coordination of the reticle layout to get these exact positions in the reticle field and the inspection tool sensitivity is affected by the pitch.
What is needed, therefore, is a cost-effective way to test and optimize sensitivity of inspections tools that leverages the aforementioned benefits of both array mode and random mode inspections.